The present invention relates to a method for manufacturing a substrate for presensitized plates for use in making lithographic printing plates.
Up to now, there have widely been employed aluminum plates as substrates for making presensitized plates for use in making lithographic printing plates (hereinafter referred to as "PS plates") and it has been known that the substrates and hence the aluminum plates for PS plates have been subjected to a surface-roughening treatment, i.e., so-called surface graining for the purposes of improving the adhesion between the substrate and the light-sensitive layer and of imparting water retention characteristics to the non-image areas of the resulting lithographic printing plate.
As specific means for such surface graining, there have been known, for instance, mechanical graining methods such as sand blasting, ball graining, wire graining, brush graining in which a nylon brush and an abrasive/water slurry are used, and honing-graining in which an abrasive/water slurry is blown on the surface to be treated at a high pressure; and chemical graining methods such as those in which the surface to be treated is roughened by treating it with an etching agent such as an alkali, an acid or a mixture thereof. In addition to the foregoing methods, there have also been known, for instance, an electrochemical graining method as disclosed in Japanese Patent Unexamined Publication (hereunder referred to as "J.P. KOKAI") Nos. Sho 54-146234 and Sho 48-28123; a method comprising a combination of mechanical graining and electrochemical graining methods as disclosed in J.P. KOKAI Nos. Sho 54-123204 and Sho 54-63902; and a method comprising a combination of a mechanical graining method with a chemical graining method which comprises chemically etching the surface with a saturated aqueous solution of an aluminum salt of a mineral acid as disclosed in J.P. KOKAI No. Sho 56-55291 (U.S. Pat. No. 4,242,417).
Among these surface-roughening methods, electrolytic surface-roughening treatments are favorably adopted in the present invention because the control of the conditions of the roughened surface is easy and a finely roughened surface can be obtained.
The roughened aluminum surface as such is very soft and hence easily worn out. For this reason, the surface-roughened aluminum is anodized to form an oxidized layer and a light-sensitive layer is applied onto the oxidized layer thus formed. The surface of the aluminum plate thus treated is very hard, excellent in wear resistance and shows good hydrophilicity and has good water retention characteristics and strong adhesion to the light-sensitive layer.
However, if the electrolytically surface-roughened aluminum plate is anodized immediately after the electrolytic graining, the resulting anodized layer is blackened since smut formed during the electrolytic graining easily adheres to the electrolytically roughened surface. This substantially impairs the appearance thereof, leads to the occurrence of scatter and reduction of the sensitivity of the light-sensitive layer applied thereto and further impairs the plate-examining characteristics after the development of an exposed PS plate. In addition, the printing durability of the resulting lithographic printing plate is also lowered because of the formation of a layer having poor adhesion to the light-sensitive layer on the aluminum plate.
As a method which can solve such a problem to some extent, J.P. KOKAI No. Sho 53-12739 discloses a method comprising desmutting the aluminum plate after the electrolytic surface-graining. This method makes it possible to remove smut formed during the electrolytic surface-graining, but it takes a long time period for removing unevenness of the surface due to the electrolytic surface graining treatment thereof. Moreover, J.P. KOKAI No. Sho 48-28123 discloses a method which comprises electrolytically surface-roughening an aluminum plate in hydrochloric acid as an electrolyte and then etching the plate with an alkali. In this alkali-etching treatment, the removal of the smut formed as well as the etching proceed at a high speed. Thus, if the surface which is electrolytically surface-roughened in hydrochloric acid is subjected to this alkali etching treatment, the structure of the grained surface becomes excessively smooth. As a result, the adhesion between the light-sensitive layer and the treated surface is greatly lowered and hence the printing durability of the resulting lithographic printing plate is also lowered. On the other hand, the non-image areas of the resulting printing plate are very excellent in resistance to background contamination.
In addition, J.P. KOKAI No. Sho 56-139700 disclosed a method which comprises electrolytically graining the surface of a substrate (an aluminum plate) for printing plates in a dilute aqueous acid solution or an aqueous salt solution, etching the surface with an alkaline aqueous solution in an amount ranging from 0.4 to 3.0 g/m.sup.2 and then anodizing the surface. The inventors of this invention have conducted detailed investigation of this method and as a result they have found that the printing durability is inevitably reduced if the surface is etched in such an amount of not less than 0.4 g/m.sup.2.
Further, J.P. KOKAI No. Sho 56-47041 (U.S. Pat. No. 4,824,757) discloses a method comprising the steps of electrolytically roughening the surface of a substrate (an aluminum plate) in a nitric acid-containing electrolyte, then etching the substrate with an alkali, anodizing the substrate and applying a light-sensitive layer comprising an o-quinonediazide and J.P. KOKAI No. Sho 60-8091discloses, for instance, a method which comprises the steps of electrolytically surface-roughening a substrate (an aluminum plate) in an electrolyte containing nitric acid or a salt thereof, etching the substrate with an alkali or an acid so that the amount of the aluminum removed is equal to 0.1 to 0.5 g/m.sup.2, then anodizing the substrate and applying a negative-working light-sensitive layer.
The inventors of this invention have also conducted detailed investigation of these improved methods and as a result they have found that a severe scatter of the printing durability of the resulting lithographic printing plates is observed and thus only lithographic printing plates showing unstable quality are obtained according to these improved methods.